Apparatus for obtaining a dross-free crystalline growth melt

ABSTRACT

THIS INVENTION PROVIDES APPARATUS FOR GROWING WEBBED DENDRITIC SEMICONDUCTOR MATERIAL FROM A MELT SUBSTANTIALLY FREE OF ANY DROSS ON THE SURFACE OF THE MELT. THE APPARATUS COMPRISES A TWO COMPONENT CRUCIBLE, CONSISTING OF AN INNER SHELL AND AN OUTER SHELL. THE MELT IS FORMED IN THE OUTER SHELL. THE INNER SHELL, AXIALLY ALIGNED WITH THE OUTER SHELL, HAS A VERTICAL DISPLACEMENT MEANS ATTACHED TO IT. THE INNER SHELL IS LOWERED INTO THE MELT OF THE OUTER SHELL AND DROSS FREE MELT IS FORCED THROUGH AN APERTURE IN THE APEX OF A CAVITY OF THE INNER SHELL FORMING A POOL OF DROSS FREE MELT THEREIN. FROM THIS DROSS FREE MELT THE WEBBED DENDRITIC SEMICONDUCTOR MATERIAL IS GROWN.

I March 2, 1971 s. OHARIA APPARATUS FOR oBTAINiNe A DROSS-FREECRYSTALLINE GROWTH MELT FilecLOct. 11, 1967 FIG. 2"

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United States Patent U.S. Cl. 23273 7 Claims ABSTRACT OF THE DISCLOSUREThis invention provides apparatus for growing webbed dendriticsemiconductor material from a melt substantially free of any dross onthe surface of the melt. The apparatus comprises a two componentcrucible, consisting of an inner shell and an outer shell. The melt isformed in the outer shell. The inner shell, axially aligned with theouter shell, has a vertical displacement means attached to it. The innershell is lowered into the melt of the outer shell and dross free melt isforced through an aperture in the apex of a cavity of the inner shellforming a pool of dross free melt therein. From this dross free melt thewebbed dendritic semiconductor material is grown.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates to apparatus for producing a dross-free crystaline growth melt.

(2) Description of the prior art The process of producing webbeddendritic growths of semiconductor materials is hampered by theformation of a scum, or dross, on the surface of the melt from which theweb is grown. As a result the quality of the surfaces of the web ispoor.

To prevent the formation of scum, or dross, on the melt, purely chemicalapproaches such, for example, as the use of atmospheres of very purehydrogen and carbon monoxide during the growth of indium antimonidewebbed dendritic material have been tried, but the use of selectiveatmosphere has not always reduced the scum formation.

SUMMARY OF THE INVENTION This invention provides apparatus for growingwebbed dendritic semiconductor material comprising a crucible consistingof an outer shell member and an inner shell member, the shell membersbeing axially aligned with respect to each other, a cavity disposed inthe top surface of the outer shell member, the apex of the cavitycoinciding with the aligned axis, the inner shell member consisting of abowl-shaped member, the apex of the bowl coinciding with the alignedaxis, an outwardly extending brim integral with the peripheral edge ofthe bowl, an aperture axially disposed in the apex portion of the bowl,means for displacing the inner shell member vertically along the alignedaxis, and means for heating the crucible.

An object of this invention is to provide a melt of semiconductormaterial for the growth of webbed dendritic semiconductor materialwherein the surface of the melt is essentially free of the formation ofdross thereon.

Another object of this invention is to provide apparatus for growingwebbed dendritic semiconductor material wherein the melt is contained ina two component crucible suitable for providing a dross free melt forgrowth therefrom.

Another object of this invention is to provide apparatus for growingwebbed dendritic semiconductor material 3,567,397 Patented Mar. 2, 1971in which means are provided to control the thermal gradient in the meltand in the web being grown.

Other objects will, in part, be obvious and will, in part, appearhereinafter.

DRAWINGS For a better understanding of the nature and objects of thisinvention reference should be made to the following detailed drawings,in which:

FIG. 1 is a view, partly in cross-section of apparatus suitable forgrowing webbed dendritic semiconductor material from a dross-free melt;and

FIG. 2 is a view in cross-section of a two component crucible made inaccordance with the teachings of this invention.

DESCRIPTION OF THE INVENTION With reference to FIG. 1, there is shownapparatus 10 for growing dendrites of semiconductor material. Theapparatus 10 comprises a two-piece crucible 12 disposed Within a quartzshield 14. The crucible 12 consists of an outside shell member 16 and aninside shell member 18. The members 16 and 18 are each made of asuitable material, such, for example, as quartz, platinum and graphite.The shield 14 serves both as a means to contain a preferred environmentand also as a heat reflector.

The outside shell member 16 may be supported within the shield 14 by asupport 20. The member 16 has a passage 22 contained therein for theinsertion of a temperature sensing device for determining thetemperature of a melt disposed within a cavity 24 of the member 16. Thecavity 24 has a curved surface 26 and is centrally disposed in the uppersurface of the member 16-.

The inside shell member 18 has a cavity 28, an outer surface 30 of whichis curved and conforms to the curvature of the surface 26 of the outsideshell member 16. The inside shell member 18 has an integral outwardlyextending brim 32 which permits the surface 30 to become contiguous withthe surface 26 when the member 18 has been lowered as far as possibleinto the cavity 24. Two or more supports 34 may be affixed to the brim32 to provide means for the vertical displacement of the member 18 asrequired. A passageway 36 is provided in the center of the surface 30 toprovide a means for liquified material to flow from one side of thecavity 28 to the other side of the cavity 28.

The crucible 12 is heated by a suitable means, such, for example, as aninduction coil 38 disposed about the outer periphery of the shield 14.Preferably the crucible 12 is heated in a preferred atmosphere.Therefore, the shield 14 may have end plates of suitable materialenclosing the crucible therein and a means for providing the necessaryatmosphere within the enclosed shield 14.

With reference to FIG. 2, there is shown the outer shell member -16containing a melt 38 of semiconductor material in the cavity 28. On thesurface of the melt is a layer 40 of dross. The presence of this drosslayer 40 gives rise to poor quality surfaces on webbed dendriticsemiconductor material grown from the melt 38.

The inner shell member 18 is lowered into the melt 38 within the cavity24. As the surface 30 of the member 18 enters the melt 38, it pushesaside substantially all of the layer 40 of dross and a portion of themelt 38 is forced through the passageway 36 and into the cavity 28.During the growth of webbed dendritic material from the melt 38 in thecavity 28, a layer of dross will also form on the original dross freesurface. When the dross becomes intolerable, it is only necessary tostop the growth process, remove the inner shell member 18 and replace itwith a clean member. The dendritic growth process can then be resumed.The inner member 18 and the outer member 16 should be so constructed asto enable one to have a depth of melt of at least one half inch in thecenter of the cavity 28 and a surface of one inch minimum diameter.

Another cause of undesirable surface defects on the webbed dendriticsemiconductor material being grown is generally attributable to anexcessive temperature gradient along a short length of the Web as itemerges from the melt 38 as well as sub-surface defects, such, forexample, as discontinuities in the twin planes, which are attributableto thermal conditions in the crystal growth regions of the melt 38.

To minimize the effect, the inner shell member 18 has on the thermalconditions of the melt 38, it is desirable to make the walls of thecavity 28 of the member 18 as thin as possible. This minimizes thethermal relaxation which occurs and consequently the sufllciently shortperiod of thermal relaxation which occurs does not give rise to hunting(dimensional oscillation) of the webbed dendritic structure being grown.The wall of the cavity 28 of the member 18 is preferably of an inch inthickness.

When the outer shell member 18 is fully submerged in the melt 38, thetemperature gradients which occur in the melt are relatively small.Consequently, the webbed dendritic material grown has large dendritesbut only a very thin 'web of material extends between them. In someinstances, no web growth occurs, only large dendrites with a voidbetween them. This latter effect arises from the fact that when a smallgradient exists, relatively less heat may be discharged through themelt, and the dendrites which grow deeper in the melt, grow inpreference to the web, which is growing at a comparatively higherlocation.

Almost complete removal of the member 18 from the melt 38 producessteeper temperature gradients within the melt 38. As a result of thissupercooling of the melt 38, the surface of the melt 38 is coolerthereby supporting the growth of thicker webs.

Additionally, the walls of the member 18 provide a means for partiallycontrolling the thermal conditions within the cavity 28 at a pointimmediately above the surface of the melt in the webbed dendriticmaterial as it is withdrawn from the melt. The walls help to maintain athermal gradient in the webbed dendritic material which is mostconducive to producing an improved quality of webbed material.Additionally, that portion of the induction coil 38 which extends abovethe crucible 12 during the growth of material may be separated from themain body of the coil 38 and separately controlled as to position andpower. This separate controlling of this portion of the coil 38 willallow the operator too have a better control of the thermal gradientwithin the web being grown.

The result of practicing this invention has provided webbed dendriticsemiconductor material having excellent semiconductor quality and alsosubstantially free of surface imperfections previously encountered inmaterial produced in prior art apparatus.

While the invention has been described with particular reference tospecific embodiments and examples of the invention, it will beunderstood, of course, that modifications, substitutes and the like maybe made therein without departing from its scope.

I claim as my invention:

1. Apparatus for growing semiconductor material from a melt comprising acrucible comprising an outer shell member and an inner shell memberaxially aligned with respect to each other and being so dimensioned thatduring crystal growing from a melt the inner shell member floats in themelt of the semiconductor material and is separated by a layer of themelt from the walls of the outer shell member during crystal growing;

said outer shell member having walls defining a cupshaped cavity withthe open end of the cavity opening into the top surface of the member;

a freely separable bowl shaped member having walls about of an inch inthickness forming said inner shell member and having an aperturedisposed in the lower apex portion of said bowl, said apex coincidingwith said aligned axes, said thin walls minimizing the thermalrelaxation of a melt when growing materiall therefrom;

an outwardly extending brim integral with the peripheral edge of saidWalls of said inner member;

means attached to said inner shell member for displacing said innershell member vertically along the aligned axes, the inner and outershells being so constructed and arranged that during crystal growing,semiconductor material is fed into the space between the shells anddross floats to the top of such space while pure material flows throughthe aperture to the inner shell; and

means for heating the crucible.

2. Apparatus of claim 1 in which the material comprising the crucible isone selected from the group consisting of quartz, platinum and graphite.

3. Apparatus of claim 1 in which the outer shell member, has temperaturesensing means disposed therein.

4. Apparatus of claim 1 in which the outer surface of the bowl of saidinner member conforms to the inner surface of the cavity of the outershell member.

' 5. Apparatus of claim 1 in which the means for heating the crucible isan electrical resistance coil.

6. Apparatus of claim 5 in which the electrical resistance coil isdivided into separate portions, each portion having a separate controlmeans, one portion of said coil being suitable to control the thermalgradient of the material grown by use of the apparatus.

7. Apparatus of claim 6 in which said one portion of said coil tocontrol the thermal gradient of the material grown is verticallydisplaced above the other portion of said coil and about a portion ofthe apparatus above the' outer shell member.

References Cited UNITED STATES PATENTS 2,,892,739 6/1959 Rusler 232732,944,875 7/1960 Leverton 23-273 2,956,863 10/1960 Goorissen 232733,041,133 6/1962 Hicks et al. 23273 3,058,915 10/1962 Bennett 232733,154,384 10/1964 Jones 23-273 3,291,571 12/1966 Dohmen et al. 23-273WILBUR L. BASCOMB, 111., Primary Examiner R. T. FOSTER, AssistantExaminer US. Cl. X.R. 24; 148173

